The movie in general. Juno is portrayed as being generally savvy, self-and-others aware, and pretty much the smartest, coolest one in the room. Why, then didn't she have the sense to use birth control? Surely she knows where babies come from! (ps—yeah, the same reason Hamlet didn't kill his usurping uncle four or five times over when he had the chance—there wouldn't have been a story. But still...).
Heat of the moment, perhaps? Intelligent people are just as subject to their hormones as anyone, and despite what the condom pushers say, people are not rational enough to consider the consequences when they let their sexual desires take control. Also, smart people take bad advice from their peers about as often as anyone else. Maybe one of her friends who's good at statistics but inept at logic happened to tell her about how low one's chances of getting pregnant from having sex just once are (and neglected to mention how that chance is still a chance and people rarely have sex just once)?
Maybe she just acts savvy, smart, and cool to divert attention from the fact she's an awkward, unpopular, pregnant teenager, and/or we're supposed to think she suddenly wised up upon seeing the consequences of her irresponsible behavior. And, anyway, she's witty but not necessarily sensible. She doesn't make that many smart decisions totally on her own.
Being self aware and/or smart doesn't mean impervious to stupid decisions. While she is quick witted there are many scenes showing that she is still a kid with a kid viewpoint of the world. Brenda criticizing her for visiting Mark with the ultrasound shows that she really isn't as smart as much as she would like to believe.
Their teenage daughter is pregnant and the parents are apparently COOL WITH IT. Not angry, seriously disappointed, depressed or humiliated (yes...that emotion might come up). They are just sort of blase about their teenaged daughter being impregnated. And...no serious discussions about the fact that their daughter getting pregnant means she is probably having unprotected sex.
Well both of her parents seem rather understanding, and really, unless they're gonna drag her to an abortion clinic or a surgeon to restore her hymen, what can they really do about it? Juno intended to give up the child off the bat and seemed to be acting responsible for her actions. Plus we don't really see most of the pregnancy as it skips ahead several months a few times.
There was one point where Bren reminds Juno that she has made sacrifices for her. Granted they specifically mention her wanting to own dogs, but the implication seemed to be there (particularly since she was sewing maternity pants for Juno).
There are actually parents in the world who would be pretty cool-headed about such situations. The usual reaction might be an extremely negative one, but the way Juno's parents responded seemed pretty in-line with their personalities (especially since Juno immediately told them she was going to give the baby up for adoption). Her dad also delivers a line - saying that he thought Juno knew when to say when - that does have a noticeable hint of anger in it, so evidently while he can keep his cool he's clearly displeased.
I hated how Jason Bateman's character suddenly turns from nice guy to nigh-pedophile douche in the span of a single scene. Especially since he was hoping to raise a child with his wife. Why would he suddenly decide to get a divorce, move out, and get involved with a teenage girl? It made no sense other than to add drama.
He never seemed as enthusiastic about raising a kid as she was. And Juno made him feel like a "cool rock star" again, instead of "grown man who is about to become a father and is going to be making jingles instead of albums for the rest of his life". It was pretty foreshadowed the moment she first showed up with Vanessa not at home, as Juno's stepmom points out. All of the stuff he liked and enjoyed was hidden away into one room while his wife decorates the house to look like a toothpaste ad. He just didn't like it and saw Juno as a chance to escape.
Wait, you seriously didn't see his entire character arc coming from the moment he and Juno snuck into his room? That wasn't even foreshadowing, that was 256-shadowing (if you'll excuse the terrible pun).
This. The guy doesn't even sound all that enthused when he's first introduced and they're talking about the baby, and when she shows him the ultrasound scan, 'would ya look at that?' isn't exactly the first thing I'd expect to hear from a man excited to become a father. The fact that everything's not quite right with Mark and Vanessa is pretty much stated from their very first scene.
Juno herself didn't realize he was heading that way until that scene, and the film is, mostly, from her point of view. The signs are there, but she either ignores them or doesn't see them.
This is actually about the page on this site, but WHO is Hobson? I've seen this movie half a dozen times and never noticed a character with that name, yet they're listed at least twice as Deadpan Snarker.
The guy at the convenience store played by Rainn Wilson.